1. Field of the Invention
The present invention relates to a helical antenna for wireless communication, and more particularly relates to a small helical antenna with a broad fan radiation pattern for a mobile terminal in mobile satellite communication or ground mobile communication and the like.
2. Description of the Related Art
A conventional helical antenna is disclosed in Japanese Published Unexamined Patent Application No. 8-78945 (78945/1996). FIG. 7 shows a perspective view of this helical antenna at 100.
The helical antenna 100 according to the prior art comprises a dielectric cylinder 104 and a flexible printed wiring sheet 107, which is wound around the dielectric cylinder 104, and is equipped with two helical balanced conductors 101 and 101'.
An unbalanced RP signal (Radio Frequency signal) in a coaxial cable 105 is converted to a balanced RP signal by a balun 108.
After that, the balanced RF signal is fed to each of the two helical balanced conductors 101 and 101'.
FIG. 8 shows an assembly procedure of the helical antenna 100 shown in FIG. 7. As shown in FIG. 8, the two balanced helical conductors 101 and 101' are adhered to the flexible printed wiring sheet 107 by a pressure sensitive adhesive double coated tape 103.
FIG. 9 illustrates a perspective view of a metal conductor 106 of the helical antenna 100 shown in FIG. 7. The end portions of the helical conductors 101 and 101' are short-circuited by a straight metal conductor 106. The metal conductor 106 secures the helical conductors 101 and 101' to enhance their mechanical strength and achieves an impedance matching of the helical antenna 100.
FIG. 10 illustrates a perspective view of the metal conductor 106 of another shape. That is, the shape of the metal conductor 106 shown in FIG. 10 is bent and suitable for achieving the impedance matching. In this case, the impedance matching of metal conductor 106 can be done comparatively easily by changing or adjusting the shape of its bent part.
In the above description, the two types of the metal conductor 106 shown in FIGS. 9 and 10 are preferred mainly for easy impedance matching and strong mechanical strength.
However, the helical antenna 100 of the prior art is not necessarily able to provide feeder impedance matching for all the helical conductors.
That is, the helical antenna 100 of the prior art is very effective for a helical antenna having a comparatively long helical conductor with two or more turns. However, in the case of a helical antenna having a broad fan radiation pattern for the mobile terminal etc., usually, the helical conductors 101 and 101' each have a length of only 1.5.lambda. (.lambda. is a wavelength of an operating frequency) and their number of turns is two or less. In this case, the feeder impedance frequency bands of the helical conductors 101 and 101' which are capable of adjusting the impedance matching by the metal conductor are very narrow. As a result, it is impossible to achieve the feeder impedance matching of the helical antenna 100 in a wide frequency band.